Generally, emulsion type compositions for skin external application contain an emulsifier in an amount of about 20 wt % based on the weight of oil. In addition, the compositions contain various skin conditioning agents, thickeners, preservatives, antioxidants, fragrances, etc. Because oil is highly effective in imparting moisturization to the skin, various efforts have been made to increase the content of oil in the compositions.
Methods that have been widely used to increase the content of oil in the prior art include a method of increasing the content of an emulsifier, and a method of increasing the content of a thickener. More specifically, in the method of increasing the content of an emulsifier, the content of an emulsifier functioning to prevent the separation between oil and water for a certain period of time is increased so that an increased amount of oil will be emulsified. In the method of increasing the content of a thickener, the viscosity of an emulsion is increased to prevent emulsion particles from moving by external environments such as temperature and heat, so that the formulation of the emulsion can be stably maintained for a long period of time while it contains a large amount of oil. The method that increases the emulsifier content enables easy emulsification, but has a shortcoming in that the emulsifier can cause skin irritation. On the other hand, the method that increases the thickener content has a disadvantage in that the emulsion is difficult to contain a large amount of oil, compared to the method that increases the emulsifier content, but has an advantage in that the emulsion is stably maintained for a long period of time.
When emulsification occurs, emulsion layers formed between the oil phase and the water phase are self-assembled by the geometrical structure of components thereof and the interaction between functional groups present in the molecules so as to minimize energy. Such self-assembly structures include micellar, hexagonal, cubic, lamellar, inverse cubic, inverse hexagonal and inverse micellar structures. Among them, the lamellar structure having a repeated multi-layer structure has an advantage, particularly in terms of stability, and is a skin-friendly structure that can be found in lipids between human keratinocytes. The lipid components that are used herein have no emulsifying function by themselves, but assist in the formation of lamellar self-assembly structures and increase the stability of formed emulsion layers.
In conclusion, it can also be contemplated to obtain a stable emulsion using lipid components other than emulsifiers or thickeners. The lipid components discussed herein have weak amphiphilic properties and are considered to have properties intermediate between the properties of surfactants and the properties of oil.
Thus, in the case of various multi-lamellar emulsions according to the prior art, stable compositions for skin external application are prepared using these emulsions in combination with lipid components that make the emulsion structures stable. However, in all these compositions, the content of lipid components, including long-chain amide, sterol, fatty acid and fatty alcohol, in the formulation, is generally 4 wt % or less, or at most 6 wt %, or 7.5 wt % or less even in specific cases, and the content of long-chain amide and sterol is also generally 1.3 wt % or less, or about 2.0 wt % even in specific cases.
Formulations containing about 15 wt % or more, particularly 15 wt % or more, of the lipid components, and about 3-4 wt % of long-chain amide and sterol, have advantages in that these are easy to transport for export or the like or store, are easily processed into final products, and can also be used as final products in some cases. Despite such advantages, there is a problem in that it is very difficult to form a stable multi-lamellar structure in conventional formulations.